The present invention is related to an optical coupler using a grating filter in which a grating (periodic perturbation of refractive index) is formed in a core of an optical fiber and a portion adjacent the core.
The present invention contributes in the field of optical communications. It is useful technology when only specific wavelength is separated out or added in wavelength multiplex transmission.
A conventional optical transmission type filter is described below.
Since it is generally common for the transmission optical filter to be required in the field of optical communications, an optical fiber grating filter is used in combination with optical parts including an optical circulator in order to transmit a predetermined wavelength band. FIG. 8 illustrates a schematic diagram of an optical fiber grating filter 22. Moreover, FIG. 9 illustrates the general reflective properties of the optical fiber grating filter 22. The optical fiber grating filter 22 formed in an optical fiber 21 is a filter of a reflection type which reflects light of a predetermined wavelength, allows transmission of other wavelengths, and is excellent in wavelength selection properties for narrow band use, and has little insertion loss. Although a usual reflective bandwidth is approximately 1 nm, if a special process is used, it can also realize a reflective bandwidth of 0.2 mm to 10 mm.
FIG. 10 illustrates an example an optical transmission filter formed by combining an optical fiber grating filter 32 and optical circulator 33 in the course of an optical fiber 31. Although a signal incident from a port 1 is. outputted to a port 2 if the optical fiber grating filter 32 is formed on the route to the port 2, only a specific wavelength (it is called wavelength xcexB hereupon) is reflected and outputted from a port 3 after it returns back to the optical circulator 33 side. If two or more signals by which wavelength multiplex is carried out from the port 1, only a signal corresponding to wavelength xcexB will be outputted from the port 3 and all the signals corresponding to other wavelength will be outputted from the port 2. That is, the multiplex separation of the signal of specific wavelength LB can be carried out.
Moreover, FIG. 11 illustrates optical transmission filter formed by combining an optical fiber grating filter 42 and an optical fiber coupler 43 in the course of an optical fiber 41. In this example, the signal inputted from the port 1 is outputted to the ports 3 and 4 by halves by the optical fiber coupler 43. At the port 3 the signal is reflected by the optical fiber grating filter 42 and the signal corresponding to wavelength xcexB reinputs to the optical fiber coupler 43, and is outputted to the ports 1 and 2 by halves again. Therefore, one fourth of the inputted signals will be outputted in the port 2. If two or more signals are inputted to the port 1 by which wavelength multiplex is done, only the signal corresponding to wavelength xcexB will be outputted from the port 2, and all the signals corresponding to other wavelength will be outputted from the port 3.
In addition, FIG. 12 shows a grating filter 52 formed in extension part of an optical fiber coupler 53 which functions as an optical transmission filter in the course of an optical fiber 51. In this example, a signal input to port 1 of the optical fiber coupler 53 is reflected by a grating filter 52 is formed in an extension portion of the optical fiber coupler 53, if the signal is of specific wavelength xcexB, and will be outputted to the port 2. In this case, if two or more signals by which wavelength multiplex is done are inputted, the signal corresponding to the wavelength xcexB is outputted to the port 2, all signal corresponding to another wavelength is outputted to the port 4 and the multiplex separation of the signal of specific wavelength xcexB can be done.
The problem in the above-mentioned conventional technology is as follows.
At first, in the method shown in FIG. 10, in case of using the combination of the optical fiber grating filter 32 and the optical circulator 33, although the insertion loss is in approximately 2 dB between the port 1 and port 2, it is excelled in properties, the problem is that the optical circulator 33 is expensive. Moreover, in the method shown in FIG. 11 by the combination of the optical fiber grating filter 42 and optical fiber coupler 43, if the optical fiber coupler 43 is compared with the optical circulator 33, although it is a cheap device, as for insertion loss, the minimum is 6 dB (namely, xc2xc). Furthermore, a signal outputted from the port 3 has an insertion loss of 3 dB (namely, xc2xd).
Furthermore, by the combination of the optical fiber grating filter 52 and optical fiber coupler 53 shown in FIG. 12, the signal of specific wavelength xcexB that is outputted from the port 2 to the port 1 and 4, the signal of specific wavelength xcexB outputted has a 0.4 dB (namely, {fraction (9/10)}) insertion loss.
In light of the forgoing, a multiplexer has a grating built-in type optical coupler and an optical amplifier, and effects not only multiplex separation of a signal but addition of a specific wavelength, and the excitation light which became unnecessary can be removed effectively and it is enabled to suppress a fall of the signal light efficiency. Furthermore, since the device does not use optical parts, such as the above-mentioned optical circulator, and is formed by an optical fiber, transmission is good. Accordingly it is an object of the present invention to provide an inexpensive wavelength multiplex transmission system having a low insertion loss.
According to the invention, there is provided an add-drop multiplexer with signal amplification ability, comprising two optical couplers of a grating built-in type with the two same structures and two optical amplifiers.
Moreover, the optical coupler of the grating built-in type of the present invention including molten extended portion of fiber coupler formed as a fiber grating, the pitch of the grating is a uniform structure in the length direction, apodization is carried out, and grating length is 2.0 mm and change in induced refractive index is 0.001. Here, apodization uses a window function for induced refractive index change of grating in the length direction of an optical fiber as shown in FIG. 13.